This disclosure relates to systems and methods for neutron measurement analysis of subterranean formations. More particularly, the disclosure relates to fast forward neutron models for the interpretation of measurements made in subterranean wells.
In neutron well logging, Monte Carlo methods are preferred for simulation of neutron well logging instrument response because of their accuracy. However, Monte Carlo methods have not been suitable for real-time neutron well logging instrument analysis due to the limited computational speed of present computers.
An example of a linear-based, Monte Carlo forward modeling technique for nuclear well logging instruments is described in Charles C. Watson, Monte Carlo Computation of Differential Sensitivity Functions, Trans. Am. Nucl. Soc., vol. 46, page 655, 1984, and Charles C. Watson, A Spatial Sensitivity Analysis Technique for Neutron and Gamma-Ray Measurements, Trans. Am. Nucl. Soc., vol. 65 (Suppl.1), pp.3-4, 1992, hereinafter referred to as “the Watson Papers.” By linearly modeling the dominant gamma-ray interactions of Compton scattering and photoelectric absorption, the technique described in the Watson papers may be used to predict the detector response of a Compton-scatter gamma ray density well logging instrument. The primary advantage of the foregoing method as applied to density instruments is its very fast computational speed, in that it can provide calculated results on a sub-second scale. The basic premise of the model assumes a linear relationship between the instrument's detector response and changes in the formation density and photoelectric properties of the formation being examined. Space around the instrument disposed in a wellbore is divided into a grid of cells, each of which is assigned a sensitivity. The contribution of each cell to the overall instrument response estimate may be obtained from pre-calculated spatial sensitivity maps. Further details of the density sensitivity function technique are disclosed in U.S. Pat. No. 5,334,833 issued to Case et al. The performance of the foregoing linear technique may include relative accuracies of a few percent in count space which must then be converted to density space. For example, when applying the linear modeling method to a logging while drilling (“LWD”) formation density measuring instrument, e.g., one operated under the trademark VISION 475, which is a mark of Schlumberger Technology Corporation, Sugar Land, Tex., USA, modeling error in comparison to experimental data was found to be as much as 0.1 g/cc within 1″ water standoff when covering typical spatial variations in density from 1 to 3 g/cc. The limited performance of the linear method is also apparent in that the density sensitivity functions are not identical when calculated using different reference formations.
Some improvement in accuracy of the foregoing method can be obtained by modifying the sensitivity functions on a case-by-case basis, but such adjustments are not fully general. A. Mendoza, C. Tomes-Verdin, and W. Preeg, Rapid Simulation of Borehole Nuclear Measurements With Approximate Spatial flux-Scattering Functions, (SPWLA 48th Annual Logging Symposium, Jun. 3-6, 2007) discloses a spatial flux-scattering functions (FSF) technique to rapidly simulate neutron porosity and gamma-gamma (Compton scattering) density well logs.
The development of the FSF technique (See also Mendoza, A., C. Torres-Verdin, and W. E. Preeg, 2010, Linear iterative refinement method for the rapid simulation of borehole nuclear measurements, Part I:Vertical wells: Geophysics, 75, no. 1. E9-E29) suggested that the technique could be applied to the generation of a fast forward neutron porosity model. Initial tests with a generic nuclear well logging tool model using AmBe chemical isotope source neutrons looked promising. However, upon scrutiny, the results provided by a FSF-based fast model for a neutron logging tool with a pulsed neutron source (which generates 14 MeV neutrons) proved inadequate in simulating results for invasion of gas-saturated formations and for handling the effects of variation of formation water salinity.
What is needed is an improved technique for fast forward modeling of neutron well logging instrument response.